Naturally occurring or crude bitumen is a sticky, tar-like form of petroleum that is so thick and heavy that it must be heated or diluted before it will flow. Bituminous sands, also known as oil sands or tar sands, are a type of petroleum deposit. Bituminous sands generally contain sand, clay, water, and crude bitumen. Bituminous sands are found in large deposits in many countries, although most notably in Canada—for example, the Athabasca Oil Sands in the province of Alberta—and Venezuela.
Bituminous sands are important as a source of oil. The bituminous sands may be extracted by strip mining or made to flow into wells by processes that reduce the viscosity of the bitumen by injecting steam or solvents into the bituminous sands deposit.
The most significant Canadian bituminous sands deposits are located in the province of Alberta: Athabasca, Cold Lake and Peace River. Between them these deposits cover over 50,000 square miles and hold reserves estimated to be approaching 2 trillion barrels of bitumen. More than 10% of this is estimated to be recoverable at current oil prices, which makes it by far the largest oil reserve in North America, and one of the largest in the world.
The largest bitumen deposit—the Athabasca deposit—comprises an area in excess of 1,000 square miles and is suitable for surface mining. The smaller Cold Lake deposits comprise some oil with a viscosity that is low enough to be extracted by conventional fluid oil extraction methods. All three bitumen deposits in Alberta are suitable for production using methods such as steam assisted gravity drainage (SAGD), described in more detail below.
In the Athabasca oil sands there are very large amounts of bitumen rich deposits covered by only a small amount of overburden, making surface mining the most efficient method of extracting the bitumen. The overburden consists of peat bog over clay and sand. The oil sands themselves are typically 40 to 60 meters deep, sitting on top of flat limestone rock. The bituminous sands are mined with power shovels and dump trucks. After excavation, hot water and caustic soda are added to crushed bituminous sand, and the resulting slurry is piped to an extraction plant where bitumen is removed. Provided that the water chemistry is appropriate to allow bitumen to separate from sand and clay, the combination of hot water and agitation releases bitumen from the oil sand, and allows small air bubbles to attach to the bitumen droplets. The bitumen froth floats to the top of separation vessels, and is further treated to remove residual water and fine solids. Bitumen is much thicker than traditional crude oil, so it must be either mixed with lighter petroleum (either liquid or gas) or chemically cracked before it can be transported by pipeline for upgrading into synthetic crude oil.
About two tons of oil sands are required to produce one barrel of oil. Originally, roughly 75% of the bitumen was recovered from the sand. However, recent enhancements to extraction methods including recovery of residual bitumen in the tailings and recovery of diluent from the froth allow for recovery of over 90% of the bitumen in the bituminous sand deposit.
Tailings in the context of oil sands mining are the residues separated out at various stages in the extraction of oil-based material from the bitumen bearing sedimentary deposits. A diluent is a lower density fluid mixed with bitumen or heavy crude to reduce the viscosity and density. Diluents are often mixed with heavy crude to allow transportation by pipeline. A common diluent used in the tar sand oil industry is natural gas condensate, which is a mixture of pentanes and heavier hydrocarbon liquids extracted from natural gas. C5 is an example of a natural gas condensate.
More details of the commercial extraction of bitumen from the tar sands are provided in FIG. 1. See D. W. Devenny, Part B Report Overview of Oil Sands Tailings Report, Figure B.12, pg. 21, http://eipa.alberta.ca/media/40994/oil%20sands%20tailings%20treatment %20technologies%20-%20final%20report%20-%20part%20b.pdf, last visited May 31, 2011. An example of extraction of the bitumen proceeds according to the following general process flow: (1) mine the bitumen containing sedimentary deposit; (2) crush the mined material; (3) form a slurry of water and crushed material; (4) pass the slurry through a coarse screen; (5) transport the slurry to an extraction plant—this hydrotransport process is also important in conditioning the slurry by separating ore particles from each other; (6) add extra water to the slurry; (7) (a) remove sand by allowing the sand to settle in a primary separation vessel (PSV) and send the sand to tailings, (b) bitumen, aided by small air bubbles, floats to the surface in the tank, forming a froth, which is removed and sent on to froth cleaning, and (c) the mixture left in the PSV, containing water, silt, clay and small amounts of bitumen is sent to middlings treatment; (8) the middlings treatment involves (a) injecting air to aid in removal of some of the remaining bitumen by forming a bitumen froth on the surface of the treatment tank, (b) collecting the froth and recycling to the PSV, and (c) sending the residual fluid to tailings; (9) bitumen containing froth is subject to (a) treatment with solvents, such as naptha, followed by (b) separation processes, such as centrifuging, to separate the bitumen from the remaining water and solids, mainly clays, and (c) the bitumen is retained for refining and (d) the separated water and solids are sent to tailings. When all of the tailings are combined, the volume of slurry exiting the extraction plant is approximately twice that of the ore containing slurry that entered the plant, and these tailings contain unrecovered bitumen, which may approach up to 10% by weight of the total bitumen content of the sedimentary deposit. Furthermore, some of the tailings currently stored in ponds, which date back to when the extraction processes were not as efficient as today, may contain more than 10% by weight of the total bitumen content of the sedimentary deposit.
After bitumen extraction, the tailings must be returned to the mine for reclamation of the land. However, currently a large fraction of the tailings have not yet been used for land reclamation—in particular the more fluid tailings, including mature fine tailings (MFTs), are stored in large tailings ponds and have proved to be very difficult to use in reclamation. MFTs are partially densified deposits formed from waste slurry from the bitumen extraction process which has a density of fine particles, primarily clay, of approximately 30%—the point at which repulsive forces between clay particles prevents further densification.
In 2009 the Canadian Energy Resources Conservation Board (ERCB) issued a directive requiring oil sand mine operators to annually increase the amounts of fluid tailings that are solidified—in order to reduce the amount of fluid tailings that are stored long-term, and to aid in increasing the rate at which the mining sites are reclaimed. In order to be suitable for reclamation the solidified tailings need to be trafficable—able to remain stable under heavy vehicle traffic, which can be quantified by a minimum undrained shear strength of 10 kPa. (This equates roughly to MFTs with a percentage weight of water reduced below 25%.) See D. W. Devenny, Part B Report Overview of Oil Sands Tailings Report, Figure B.23, pg. 36. Processing of fluid tailings to produce trafficable solids requires an efficient method of water removal from the tailings. Furthermore, water removal at earlier points in the slurry processing—during the bitumen extraction process—helps to increase the amount of fines that can be captured by sand, and thus do not end up as mature fine tails.
Clearly, there is a need for efficient methods and equipment for recovery of bitumen from sedimentary deposits and for efficient methods and equipment for dewatering of the more fluid tailings, including mature fine tails.
Open caste mining is not used when the deposits are too deep. However, other techniques may be used, some of which are described below.
Cold heavy oil production with sand (CHOPS) is a process in which oil is pumped out of the sand deposits using pumps such as progressive cavity pumps. CHOPS recovers typically around 10% of the oil from the tar sand deposits of Alberta, Calif. A large amount of sand is pumped with the oil to get the best oil recovery rates, with the disadvantage of having to dispose of the oily sand. The sand has been used to make road surfaces in parts of Canada. However, there are concerns over the effect of the residual oil on the environment, and alternative methods of disposing of or cleaning up the oily sand are needed.
Clearly, there is a need for efficient methods and equipment for recovery of oil from the oily sand left over after CHOPS, and for providing clean sand that does not pose an environmental hazard.
Steam assisted gravity drainage (SAGD) involves drilling two horizontal wells in the oil sands, one at the bottom of the bituminous sands formation and a second approximately 5 meters above it. These wells are drilled in groups from a central drilling platform and can extend for miles in all directions. In each pair of wells, steam is injected into the upper well to melt the bitumen. The bitumen flows down to the lower well and is pumped to the surface. SAGD allows high oil production rates, recovering up to 70% of the oil from the deposit and is widely used in Alberta's oil sands areas. However, the oil/water mixture that is pumped to the surface contains a large amount of suspended solids that must be removed and disposed of.
Clearly, there is a need for efficient methods and equipment for removing the suspended solids from the oil/water mixture produced by SAGD, and for providing the suspended solids in a clean form that does not pose an environmental hazard.